What are they elevating? Bridge building skills, quick sand, protected area?

Looks like a nice bridge...

I haven't yet seen the bridge, but that area has very coarse sand. This bridge appears to just be a feature, although some that have been built this year go over very loose, sandy drainage's that flash flood and get washed out.

Personally, I ride an obese bike and don't need the bridges. For some reason unknown to me though, this trail isn't about me.